Agriculture Reference
In-Depth Information
3.7
Summary
differences in transmissivity due to
orientation (north-south or east-west)
are small.
There is less uniformity of radiation
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Inside a greenhouse, radiation, temper-
ature and atmospheric composition
are modified generating a different
microclimate.
The 'greenhouse effect' is a conse-
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inside a greenhouse that is east-west
oriented than inside a north-south ori-
ented greenhouse. However, these dif-
ferences in uniformity between
multi-span greenhouses oriented east-
west and north-south are attenuated
by: (i) the greater the height of the
greenhouse; (ii) the lower the span
width; and (iii) the radiation diffusion
characteristics of plastic films used
nowadays.
Transmissivity to solar radiation must
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quence of two different phenomena:
(i) a confinement effect (convective
effect), due to the reduction in the air
exchanges with the outside atmos-
phere; and (ii) a radiative greenhouse
effect, caused by the existence of the
greenhouse cover, which is a screen
that is transparent to the Sun's rays
but has low transparency to far IR radi-
ation (emitted by the soil, the vegeta-
tion and the inner elements of the
greenhouse).
The first modification which a green-
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be maximized in the greenhouse,
because an increase in solar radiation
involves a parallel increase in yield.
This requires careful consideration
when choosing the type of greenhouse,
the covering material, and the crop
management technique (crop rows ori-
entation, plants density, pruning, train-
ing, etc.) that will be used.
The air temperature in the greenhouse
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house generates in the climate parame-
ters is a decrease of solar radiation, due
to the presence of the greenhouse
cover.
The transmissivity of a greenhouse is
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the fraction of solar radiation which
penetrates inside the greenhouse. It is
usually expressed as a percentage.
The transmissivity of the greenhouse
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is, normally, higher during the day than
the outside air temperature and similar
or slightly higher during the night than
the outside air temperature. However,
in unheated greenhouses, during clear
nights (without clouds) with no wind,
'thermal inversion' may occur (the tem-
perature of the air in the greenhouse
being lower than outside).
The plant temperature varies greatly
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depends, among other factors, on:
(i) the cloudiness; (ii) the position of the
Sun in the sky; (iii) the geometry of
the greenhouse cover; (iv) its orienta-
tion; (v) the covering material; and
(vi) the structural elements of the
greenhouse.
When direct radiation conditions pre-
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during the day (with respect to the air
temperature and between different
parts of the plant) depending on the
intercepted radiation, transpiration and
the air movements, among other factors.
The soil has great thermal inertia and
vail, on sunny days, the geometry of the
cover (shape and pitch of the roof) and
the orientation greatly influence the
greenhouse transmissivity. When dif-
fuse radiation prevails (cloudy days)
they have little influence.
In the climatic conditions of the
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during the night it returns part of the
energy it has stored during the day back
to the greenhouse. The crop, due to its
low mass, has little importance in
greenhouse thermal inertia.
The wind is very limited inside the
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Mediterranean coast, with an abun-
dance of sunny days, the east-west
greenhouse orientation transmits more
radiation in autumn-winter, than the
north-south orientation, on gabled
greenhouses with a certain roof pitch
(around 30° roof angle). If the roof pitch
is lower (around 10° roof angle) the
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greenhouse, in relation to the outside.
Therefore, it may be convenient to
move the inside air, because the lack
of air movement negatively affects
